Removal of scale deposits



3,003,899 REMOVAL OF SCALE DEPOSITS John F. Eberhard and Robert B. Rosene, Tulsa, Okla, assignors to The Dow Chemical Company, Midland, Mich a corporation-of Delaware No Drawing. Filed May 17, 1957, Ser. No. 659,744 2 Claims. (Cl. 134-22) The invention isdirected toward the removal of scale from the interior surfaces of shells, headers, connecting channels, tubes, and other vessels through which a fluid is circulated. Itis more particularlydirected toward the removal of scale from non-drainable tubes in superheaters and super-critical steam generators.

Such scale usually continues to grow during the use of the unit having such vessels, forming a coherent deposit, tenaciously adhering to the walls of the vessel, which greatly reduces the thermal conductivity of the walls of the vessels and constricts the passageway therethrough. Since the efiiciency of a heat exchange unit or nited States Patent steam boiler is improved by providing a large contacting surface about the fluid to be cooled or heated, a large number of tubes or coils of relatively small cross-section are advantageouslyemployed. The deposition of scale on such tubes is especially objectionable because the reduction in thermal conductivity of the walls of the tubes and the reduction in volume of fluid passing through the tubes results in excessively high temperatures, usually culminating in serious damage to the tubes. A further result of the constricting effect of the scale is excessive build-up of pressure, often eventually resulting in a rupture of the tube wall. H

Descaling operations employing solvents to dissolve the aforementioned scale have long been used. Particularly effective methods of descaling are described in U.S. Pat. No. 2,606,873 and U.S. pat. applications, Serial No. 599,- 952, filed July 25, 1956, and 639,623 filed February 12, 1957, which have matured into patents, Nos. 2,935,429 and 2,965,523, respectively.

The design of many heat exchange units, steam boilers, and similar industrial units requires tubes having return bends or U-shaped bends, often referred to as hair-pin bends, from which fluids cannot be completely drained unless the unit is at least partially dismantled and each tube individually treated and blown. Superheaters in steam boilers are commonly of this type. The removal of scale from such U shap'e'd or hair-pin bends has long presented a serious problem because solvents such as mineral acids, e.g., hydrochloric acid, cannot be satisfactorily removed from such bends. Known effective descaling solvents are more or less corrosive to the metals of which the tubes are made. The corrosive attack of such solvents is as: objectionablejin, thfjli sc'nce of a corrosion inhibitor when the solventwis completely removed from the tubes being treated at the end of the treatment. However, where residual descaling solvent remains in tubes over protracted periods of operation, particularly at elevated temperatures as high as 900 to 1100 C. and even higher commonly employed in superheaters, the corrosive attack of such residual, solvent is objectionable. The practice now commonly employed of removing the header plate, introducing a tightly fitting nozzle (attached to a hose carrying the solvent) into each tube individually, followed by water-flushing and usually neutralizing, is not only time consum'ing and costly but in some instances does not result in the complete removal of all treating solvent from the lower portions of the U-bends. No descaling operation is known to the inventors wherein the descaling solution can be substantially removed from non drainable tubes without such individual tube treatment. Nor is a descaling operation known to the inventors wherein thetreating solvent may ice b; permitted to remain in the tube without deleterious e ect.

, There is, therefore, a need for an improved method, of removing scale from metal surfaces which cannot be completely drained, e.g., UV-shaped superheater tubes in steam generators, whereby the tubes are rendered sub stantially free of scale without a residual corrosive substance remaining in the tubes long enough after treatment to have a deleterious effect on the tubes.

Accordingly, the principal object of the invention is to provide a method of removing scale from vessels and connecting lines through which fluid is circulated. A particular object is to provide an improved method of removing scale from non-drainable tubes.

Briefly, the invention comprises forcing an aqueous solution of an acid selected from the class consisting of formic acid and citric acid, preferably containing an inhibitor against metal attack, into tubes to be descaled, and allowing the solution to stand therein at a proper temperature for a sufficient time to remove the scale.

In practicing the invention, formic acid or citric acid is dissolved inwater to make a 0.5 to 10 weight percent solution at room temperature. Although higher percentage strength acid may be used, it has been found that there is no economic advantage in using higher than 10 .percent of ,aci ,..and 7.5 p rcent is t p rr uppe limit from the standpoint of economical operation. The 'solution is pumped into the circulation system offthe unit from which scale is to be removed and heated to between about and 212 F. for from about 1 to, 16 hours, although, where a particularly thick scale is to be removedthe heating may continue for 18 to 20 hours. The circulation system is then flushed out with water and n h t r d oi s a pin the preferred embodiment of the invention, an inhibitor to metal attack in an amount between 0.1 and 1.0 percent, and preferably about 0.4 percent, of the weight of the water is dissolved either in the water before ad'- rnixing the acid therewith or in the aqueous acid solution after admixture, Any of the known inhibitors to metal attack by acidic solutions may be employed. For ex ample, any of the organic nitrogen bases listed in Table I of U.S. Patent 2,60=6,8'Z3, preferably with an aldehyde and a wetting agent as described therein; glycol ethers of an ethanol amine as described in U.S. Patent 2,510,- 063; reaction products of a rosin amine, formaldehyde and a ketone asfdescribed in U.S. Patent 2,758,970; or the reaction products derived from cyclic bases from coal tar by a series of reactions with certain organic chlorides on the one 'hand,'and with a substance chosen from the class consisting of inorganic thiocyanates and thiourea on the other h d as esc bed n U-fi Paten 2.40 5 53.

The preferred temperature at which the aqueous solutioh is maintained is at about 200 F. A convenient wa of heating the solution in the system is by the gas burners 'orfir'e'box of the unit itself, care being takento control the heat at a suitable level for holding the temperature of the descaling solution within the desired limits. As an alternative mode of heating the solution, steam may be injeotedinto the solution in the system, allowance being made for the dilution of the solution due to condensation of the steam. Other ways of heating the solution are pumping it through a heat exchanger as it is pumped into the system to be descaled, or using hot water to prepare the aqueous-acid solution.

The length of time that the aqueous-acidic solution is maintained in the circulating system being des-caled is dependent upon the thickness and character of the scale; upon the acid being used, and the concentration and temperature of the solution. Experience in descaling units; of similar design to the one 'being descaled, after known periousof use under comparable conditions, e.g.,

3 1 I wherein water of a known similar analysis is employed, to form superheated steam at similar temperatures and pressure, will indicate the length of time that the solution should remain in contact with the scale. A particularly helpful guide to this length of time, where a section of a tube in the system requires replacement, as is often the case, is to replace such section before descaling and subject the removed section to treatment in a sample portion of the descaling solution at about the temperature employed in the descaling operation. The length of time required to loosen and dissolve the scale from the removed section of tube is then used to determine the time the descaling solution is to be kept in the system. A 1 percent aqueous formic acid solution at a temperature between 200 and 212 F. has been found ample to descale ferrous metal tubes containing appreciable amounts of scale (largely Fe O in commercial steam generators.

To show the eifectiveness of aqueous solutions of formic acid on scale deposits in boiler tubing, the tests set out in Table I were run. The formic acid solutions contained 0.4 percent of a coal tar base inhibitor as described in US. Patent 2,606,873. 1

Table 1 Percent Scale Removal Percent Length Formic of Test Acid By inHours Weight ASIM Code Steel V Corrosion Tests Percent Acid in Temp., Rate,

Water F.

11 5 pericent tormic.-..- 175 d-- 14 percent H01 175 approx. ofozo 15- do- 200 0. 500

Tests 17, 18, and 19 were run to show the eifect of temperature on the decomposition and corrosion rate on the 5 percent aqueous formic acid solution on A.I.S.I. 1020 (American Iron and Steel Institute, Specification 1020 carbon steel). Water was employed in test 16 for comparative purposes.

Table III Corrosion Rate, Pounds Per Sq. Ft. Per Day Test Solution Per Day water. v, 212 5 percent formic acid 212 do 280 0.018 -do 330 nil From Table III it is apparent that as the formic acid decomposes with increasing temperature, its corrosive effect on the metal rapidly lessens. Therefore, at well below operating temperatures of a steam generator employing superheaters, the formic acid is non-corrosive.

The decomposition products of formic acid and the products resulting from its action on scale include CO H 0, and some metallic oxides, the latter principally iron oxides as a result of the decomposition of intermediate metal formates from the scale. The decomposition temperature of aqueous ferrous formate was found to be 212 F. The decomposition temperature of formic acid as set out in Chemistry of Carbon Compounds, E. H. Rood, p. 543, is 320 F. The decomposition temperature of hydrochloric acid, on the other hand, is "1000 F. as set out in Comprehensive Treatise on Inorganic and Theoretical Chemistry, vol. 14, J. W. Iviellor, pp. 59 and 146.

To show the low rate of corrosion of aqueous solutions of citric acid containing an inhibitor, such solutions were prepared and used to treat A.I.S.I. 1020 mild carbon steel at 200 for 6 hours. The results are shown in Table IV as tests 20 and 21.

Table IV Corrosion Rate, Test Aqueous Soln 0.4 percent of Inhibitor Used Pounds Containing- Per Sq. Ft. Per Day 20 58T E2 acid Prepared according to 0.8. 0.0016

' cit c. 1 21 Epercent citric Prepared according to U18. 0.0014 acid. Pat. 2,606,873.

To show the small residue left upon decomposition of formic acid and citric acid by heat, strips of boiler superheater carbon steel were placed in 100 ml. of 3 percent aqueous solutions of the acids and the solutions heated at 110 C. for 20 minutes to evaporate to dryness and then raised to a temperature of 360 C. over a period of 15 minutes. The acid had completely decomposed at that temperature. The residues remaining are shown in Table V.

Referring to Table V above it is shown that the residue of inhibited citric and formic acid remaining is extremely small.

To ascertain the corrosive eifects on boiler tube bends of A.I.S.I. 1010 superheater steel and the amount of residue after decomposition therein of formic acid and citric acid containing no inhibitor, specimens of the superheater U-bend tubing were weighed and some filled with 10 percent formic acid and others with 10 percent citric acid. The temperature of the bends containing the solutions of formic and citric acid were raised slowly to 350 F. and maintained at that temperature for 16 hours. The bends were then reweighed. The loss in weight was attributed to corrosion and the rate of corrosion calculated as the loss per sq. ft. per day.

The results obtained are set out in Table VI below.

Table VI Loss of 10 percent Corrosion rate, Weight, Lbs. Test No. Aqueous Acid Lbs. per Sq. Per Sq. Ft. Soln Ft. Per Day of Heating Surface 0. 0064 0.0758 0. 0096 0. 0382 0. 0078 0. 0256 0. 0135 0. 0425 28 ..do 0.0202 0.0950 29 ..d 0.0176 0.0763

The corrosion rate of steel in a steam atmosphere arising from boiling solutions of formic and citric acid containing no inhibitor were ascertained by suspending sections of A.I.S.I. 1010 steel in a steam chamber above the boiling solution for 6 hours at 177 C. The results are set out in Table VII.

The following procedure illustrates a treatment of a 3000 gallon, non-drainable superheater steam generator unit in accordance with the invention. An aqueous solution of formic acid of about 1 percent strength by weight is prepared as by dissolving about 100 gallons of 8-5 percent formic acid in 2900 gallons of water. To this solution, about 0.4 percent of a coal-tar base inhibitor, prepared according to U.S. Pat. 2,606,873, or about 12 gallons, are added. The solution is heated to about 200 F.

The boiler unit to be treated is then cooled to between about 210 and 200 F. and the formic acid solution pumped into it. The temperature of the solution in the system is maintained at about 200 F. for about 2 hours. The temperature may be maintained by continuing to fire the boiler of the unit. After 2 hours, the unit is flushed twice with water. It may be desirable to follow the Water flush with a 0.5 to 1.0 percent aqueous solution of sodium carbonate.

The flushing removes a portion of the formic acid solution but some by-passing of the formic acid occurs because of the construction of the non-drainable super-heaters.

Most of the flush water passes through only a few of the numerous tubes connected to the headers through which the flushing must be conducted, the remaining tubes, therefore, being only partially flushed. Some treating solution, therefore, remains in the tubes.

The unit is then put back in operation. The heat of operation shortly thereafter decomposes any residual acid in the treating solution forming a non-corrosive residue and a volatile material. The corrosion rate of formic acid rapidly lessens from 200 to 250 F. and is negligible at 320 to 330 -F. Iron formate, for example (which is likely formed when formic acid attacks the scale) decomposes at about 210 F. and that of formic acid at between 320 and 330 F.

The instant invention provides a number of advantages in boiler scale removal among which are:

(l) A low decomposition temperature forming a noncorrosive residue in a non-drainable tube such as U-bends in superheaters.

(2) Employment of a higher scale-removing temperature, for example, 200 to 210 'F., as compared to not over for hydrochloric acid solutions.

-(3) Low cost materials and simple descaling operations.

Having described the invention, what is claimed and desired to be protected by Letters Patent is:

1. The method of removing iron oxide-containing scale from the interior of non-drainable tubes in superheaters and supercritical steam generators consisting essentially of dissolving between 0.5 and 7.5 percent by weight of an organic acid selected from the class consisting of citric acid and formic acid in water to make an aqueous descaling composition, forcing the solution thus made into contact with the scale in said non-drainable tubes, subjecting the solution thus in contact with the scale to a temperature of between 150 and 212 for a period of time of at least 1 hour, and thereafter subjecting residual acid remaining in said tubes to a temperature above 212 F. to decompose said acid and render it substantially noncorrosive.

2. The method of claim 1 wherein said residual acid is subjected to a temperature of at least about 320 F.

References Cited in the file of this patent UNITED STATES PATENTS 1,334,092 Harmeling Mar. 16,1920 2,203,649 Felkers June 4, 1940 2,318,559 Percival May 4, 1943 2,326,837 Coleman Aug. 17, 1943 2,628,199 Lowenheim Feb. 10, 1953 2,666,000 De Hofl Jan. 12, 1954 2,793,191 Streicher May 21, 1957 FOREIGN PATENTS 248,886 Great Britain Mar. 18', 1926 

1. THE METHOD OF REMOVING IRON OXIDE-CONTAINING SCALE FROM THE INTERIOR OF NON-DRINABLE TUBES IN SUPERHEATERS AND SUPERCRITICAL STEAM GENERATORS CONSISTING ESSENTIALLY OF DISSOLVING BETWEEN 0.5 AND 7.5 PERCENT BY WEIGHT OF AN ORGANIC ACID SELECTED FROM THE CLASS CONSISTING OF CITRIC ACID AND FORMIC ACID IN WATER TO MAKE AN AQUEOUS DESCALING COMPOSITION, FORCING THE SOLUTION THUS MADE INTO CONTACT WITH THE SCALE IN SAID NON-DRAINABLE TUBES, SUBJECTING THE SOLUTION THUS IN CONTACT WITH THE SCALE TO A TEMPERATURE OF BETWEEN 150* AND 212*F. FOR A PERIOD OF TIME OF AT LEAST 1 HOUR, AND THEREAFTER SUBJECTING RESIDUAL ACID REMAINING IN SAID TUBES TO A TEMPERATURE ABOVE 212* F. TO DECOMPOSE SAID ACID AND RENDER IT SUBSTANTIALLY NONCORROSIVE. 